A non-destructive inspection allows an inspector to gain information about underlying structure and potential defects within that structure without having to destroy the structure. Non-destructive techniques are used to identify both service- and manufacturing-related flaws and damage. Some examples of non-destructive techniques are "tap tests," eddy currents, and ultrasonics. A tap test has applicability to inspecting aircraft structures, and is well suited for inspecting composite and metal honeycomb structures. To perform a tap test, a tap test probe, like probe 20 shown in FIG. 1, is tapped against a structure under inspection. Generally, the less stiff a tapped structure is, the duller the sound of the tap, and also the longer the probe remains in contact with the tapped structure. Contact time--also called impact duration (.tau.)--is related to the stiffness of the tapped structure based on a grounded-spring mechanical model, which yields the following equation: EQU .tau.=.pi.(m.sub.T /k).sup.1/2 or k=m.sub.T (.pi./.tau.).sup.2
where m.sub.T and k are respectively the mass of the tap test probe and local stiffness (or spring constant) of the structure being inspected.
One known tap test device for inspecting aircraft structures is the Rapid Damage Detection Device (RD.sup.3) built and tested by the Boeing Defense & Space Group, and described in Georgeson et al., "Electronic tap hammer for composite damage assessment," SPIE, vol 2945, pp. 328-38. The RD.sup.3 is a hand-held device that consists of a hammer containing an accelerometer in the hammer's head. The hammer is connected by a cable through its handle to a hand-held module containing electronics and a liquid crystal digital display (LCD). The accelerometer in the head of the hammer translates a force-time pulse for each tap into a voltage. Georgeson et al. note that the width of this pulse is sensitive to local stiffness. The electronics in the hand-held module takes a measurement of the width of the pulse at a pre-set level of 4.8 volts. The measured pulse width is displayed as a number on the LCD display. After each hammer tap, the display resets and shows a new value. Another tap test inspection device is the Woodpecker WP632 Tapping Exfoliation Detector developed by Mitsui Engineering & Shipbuilding Co., Ltd (see also U.S. Pat. No. 5,048,320). The WP632 also processes the force-time pulse and provides a measure of contact time. The WP632 compares the measured contact time with a reference value and provides an indication of the inspection result by way of light emitting diodes that light up when an abnormality is detected. The WP632 may be connected to a separate monitoring unit for showing and memorizing measured contact time values. The monitoring unit can also transmit this data to a personal computer.
Although the RD.sup.3 and Woodpecker WP632 devices both display a measure of the width of the force-time pulse, and that width is related to stiffness by the equation identified above, neither device calculates or displays a measure of stiffness itself. Also, Cawley and Adams discuss possible ways the force-time history and/or frequency spectra of an impact may be compared to the force-time history and/or frequency spectra of a reference impact in the article "Sensitivity of the Coin-Tap Method of Nondestructive Testing," Materials Evaluation, pp. 558-63 (May 1989). Cawley and Adams note that obvious candidates are comparisons of peak force, impact duration (that is, the width of the force-time history pulse), and rate of decrease of force, but comment that all these possibilities have serious drawbacks, and in particular note that a test based on impact duration would tend to be unreliable. Cawley and Adams then describe another method, which involves obtaining the frequency spectrum of the force pulse with the use of a Fourier-transform microchip. The spectrum is analyzed to come up with a measure that approximates the degree of high-frequency force. Cawley and Adams note that impacts over defects have lower high frequency force than impacts over good areas.
Two-dimensional "C-scan" images have also been used to display non-destructive inspection results. C-scans are two-dimensional images produced by digitizing point-by-point signal variations of an interrogating sensor while it is scanned over a surface. Such displays have been provided using a conventional C-scan apparatus placed on the structure to be inspected. The C-scan apparatus carries an inspection probe across the structure taking inspection data at various points. Multiple inspection data points are thus obtained, along with corresponding location information for each datum obtained. This information is sent to a video display device where inspection results are displayed in a format that shows the inspection results arranged to correspond to the locations of the structure from which the data were obtained. A C-scan display such as this is described as having been used with a tap test device called the "Tapometer," in the article by Adams, Allen & Cawley, "Nondestructive Inspection of Composite Structures by Low-velocity Impact," in QNDE, vol. 5, 1986, Plenum Press, New York.
C-scan images have diagnostic benefits because of being able to distinguish easily between an actual defect and normal stiffness variations due to, for example, internal support structures. However, the need for a C-scan apparatus imposes portability limitations that prevent its use in many cases. For example, when inspecting an aircraft structure in a maintenance environment, it is rarely an option to remove from the aircraft the structure needed to be inspected and take it to a laboratory to perform the inspection. The same holds true for other structures typically inspected by non-destructive techniques, for example, pipelines. The C-scan apparatus also may cause the structure being tested to be scratched, either by the frame of the C-scan apparatus itself when securing the frame to the structure to be inspected or when removing it, or by the C-scan apparatus scratching the structure when moving an inspection probe from one inspection point to another. Moreover, for curved structures such as the leading edge of aircraft wings, a C-scan apparatus, with its large rigid frame, cannot be used.